Internal combustion engines are subject to strict exhaust emissions limits. Approaches to reducing emissions include new combustion designs and fuel modifications, but these improvements have fallen short of meeting the emissions limits. Other approaches involve the use of exhaust aftertreatment devices, which have achieved significant emissions reductions.
For diesel engines, which are conventionally run at a lean air-fuel ratio, the main pollutants of concern are oxides of nitrogen (NOx) and particulate matter (PM). The NOx levels result from the lean air-fuel ratio and the relatively high levels of oxygen in the exhaust. The PM is composed of black smoke (soot), sulfates generated by sulfur in fuel, and components of unburned fuel and oil.
Modern diesel engines must meet strict NOx emissions limits while also providing high energy conversion efficiency. To these ends, current diesel engines use SCR (selective catalytic reduction) or LNT (lean NOx trap) catalysts. When the engine is operated at a lean air-fuel ratio, these catalysts are fairly effective at NOx reduction without loss of engine efficiency.
A three-way catalyst (TWC) of the type used for gasoline engines is effective at NOx reduction, but does not function well with higher levels of oxygen in the exhaust. If the diesel engine is run at a stoichiometric air-fuel ratio to allow use of a TWC, engine efficiency is degraded.